gui: bugfix, getAvailableBalance skips selected coins

The previous behavior for getAvailableBalance when coin control
has selected coins was to return the sum of them. Instead, we
are currently returning the wallet's available total balance minus
the selected coins total amount.

This turns into a GUI-only issue for the "use available balance"
button when the user manually select coins in the send screen.

Reason:
We missed to update the GetAvailableBalance function to include
the coin control selected coins on #25685.

Context:
Since #25685 we skip the selected coins inside `AvailableCoins`,
the reason is that there is no need to traverse the wallet's
txes map just to get coins that can directly be fetched by
their id.

src/bench/wallet_create_tx.cpp

@@ -102,7 +102,7 @@ static void WalletCreateTx(benchmark::Bench& bench, const OutputType output_type
     }
 
     // Check available balance
-    auto bal = wallet::GetAvailableBalance(wallet); // Cache
+    auto bal = WITH_LOCK(wallet.cs_wallet, return wallet::AvailableCoins(wallet).GetTotalAmount()); // Cache
     assert(bal == 50 * COIN * (chain_size - COINBASE_MATURITY));
 
     wallet::CCoinControl coin_control;
@@ -161,7 +161,7 @@ static void AvailableCoins(benchmark::Bench& bench, const std::vector<OutputType
     }
 
     // Check available balance
-    auto bal = wallet::GetAvailableBalance(wallet); // Cache
+    auto bal = WITH_LOCK(wallet.cs_wallet, return wallet::AvailableCoins(wallet).GetTotalAmount()); // Cache
     assert(bal == 50 * COIN * (chain_size - COINBASE_MATURITY));
 
     bench.epochIterations(2).run([&] {

src/qt/sendcoinsdialog.cpp

@@ -791,8 +791,13 @@ void SendCoinsDialog::useAvailableBalance(SendCoinsEntry* entry)
     // Include watch-only for wallets without private key
     m_coin_control->fAllowWatchOnly = model->wallet().privateKeysDisabled() && !model->wallet().hasExternalSigner();
 
+    // Same behavior as send: if we have selected coins, only obtain their available balance.
+    // Copy to avoid modifying the member's data.
+    CCoinControl coin_control = *m_coin_control;
+    coin_control.m_allow_other_inputs = !coin_control.HasSelected();
+
     // Calculate available amount to send.
-    CAmount amount = model->getAvailableBalance(m_coin_control.get());
+    CAmount amount = model->getAvailableBalance(&coin_control);
     for (int i = 0; i < ui->entries->count(); ++i) {
         SendCoinsEntry* e = qobject_cast<SendCoinsEntry*>(ui->entries->itemAt(i)->widget());
         if (e && !e->isHidden() && e != entry) {

src/wallet/interfaces.cpp

@@ -18,6 +18,7 @@
 #include <util/system.h>
 #include <util/translation.h>
 #include <util/ui_change_type.h>
+#include <wallet/coincontrol.h>
 #include <wallet/context.h>
 #include <wallet/feebumper.h>
 #include <wallet/fees.h>
@@ -403,7 +404,24 @@ public:
     CAmount getBalance() override { return GetBalance(*m_wallet).m_mine_trusted; }
     CAmount getAvailableBalance(const CCoinControl& coin_control) override
     {
-        return GetAvailableBalance(*m_wallet, &coin_control);
+        LOCK(m_wallet->cs_wallet);
+        CAmount total_amount = 0;
+        // Fetch selected coins total amount
+        if (coin_control.HasSelected()) {
+            FastRandomContext rng{};
+            CoinSelectionParams params(rng);
+            // Note: for now, swallow any error.
+            if (auto res = FetchSelectedInputs(*m_wallet, coin_control, params)) {
+                total_amount += res->total_amount;
+            }
+        }
+
+        // And fetch the wallet available coins
+        if (coin_control.m_allow_other_inputs) {
+            total_amount += AvailableCoins(*m_wallet, &coin_control).GetTotalAmount();
+        }
+
+        return total_amount;
     }
     isminetype txinIsMine(const CTxIn& txin) override
     {

src/wallet/spend.cpp

@@ -356,12 +356,6 @@ CoinsResult AvailableCoinsListUnspent(const CWallet& wallet, const CCoinControl*
     return AvailableCoins(wallet, coinControl, /*feerate=*/ std::nullopt, params);
 }
 
-CAmount GetAvailableBalance(const CWallet& wallet, const CCoinControl* coinControl)
-{
-    LOCK(wallet.cs_wallet);
-    return AvailableCoins(wallet, coinControl).GetTotalAmount();
-}
-
 const CTxOut& FindNonChangeParentOutput(const CWallet& wallet, const COutPoint& outpoint)
 {
     AssertLockHeld(wallet.cs_wallet);

src/wallet/spend.h

@@ -94,8 +94,6 @@ CoinsResult AvailableCoins(const CWallet& wallet,
  */
 CoinsResult AvailableCoinsListUnspent(const CWallet& wallet, const CCoinControl* coinControl = nullptr, CoinFilterParams params = {}) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet);
 
-CAmount GetAvailableBalance(const CWallet& wallet, const CCoinControl* coinControl = nullptr);
-
 /**
  * Find non-change parent output.
  */

src/wallet/test/wallet_tests.cpp

@@ -581,7 +581,7 @@ BOOST_FIXTURE_TEST_CASE(ListCoinsTest, ListCoinsTestingSetup)
     BOOST_CHECK_EQUAL(list.begin()->second.size(), 1U);
 
     // Check initial balance from one mature coinbase transaction.
-    BOOST_CHECK_EQUAL(50 * COIN, GetAvailableBalance(*wallet));
+    BOOST_CHECK_EQUAL(50 * COIN, WITH_LOCK(wallet->cs_wallet, return AvailableCoins(*wallet).GetTotalAmount()));
 
     // Add a transaction creating a change address, and confirm ListCoins still
     // returns the coin associated with the change address underneath the